As personal computers have become more developed and widespread, electronic still cameras quickly have gained popularity as image input devices. The total pixel number of the solid-state imaging elements used in electronic still cameras has exceeded 1 million pixels, and recently electronic still cameras provided with solid-state imaging elements having a total pixel number greater than 3 million pixels also have appeared on the market. Video cameras that are capable of capturing high-quality still images in addition to moving pictures also have appeared on the market.
The optical system of electronic still cameras is provided with a taking lens, an optical low-pass filter, and a solid-state imaging element arranged in that order from the object side to the image plane side. The taking lens forms on the imaging surface of the solid-state imaging element an actual image that corresponds to an object. The solid-state imaging element carries out spatial sampling depending on the pixel structure, and outputs an image signal of the image formed on the imaging surface. The solid-state imaging element is thin, lightweight, and compact, and therefore the electronic still camera can be made compact.
Solid-state imaging elements perform spatial sampling depending on the pixel structure, however, an optical low-pass filter is typically disposed between a zoom lens, which serves as the taking lens, and the solid-state imaging element to remove the aliasing distortion that occurs at this time, removing high-frequency components from the image formed by the zoom lens. Typically, optical low-pass filters are made of a quartz plate. Here, the characteristic that is exploited is that when natural light is incident on the quartz plate, the natural light is split into an ordinary ray and an extraordinary ray due to the birefringence of the quartz, and these are emitted parallel to one another.
With solid-state imaging elements, when the pixel number is increased but the picture plane size is kept the same, the pixel pitch becomes small, lowering the aperture ratio and the photosensitivity. Accordingly, by providing a miniature positive lens at each pixel of the solid-state imaging element, the effective aperture ratio is increased, preventing a drop in the photosensitivity. In this case, to allow most of the light emitted from the miniature positive lenses to arrive at corresponding pixels, it is necessary to configure the zoom lens so that the principle rays that are incident on the pixels are substantially parallel to the optical axis. That is, there must be good telecentricity.
Although electronic still cameras come in many forms, one example is a compact type electronic still camera provided with a zoom lens having a ×2 to ×3 zoom ratio. One demand on compact type electronic still cameras is that they be easy to carry, and it is necessary that the optical total length at least when not in use (the distance from the apex of the lens surface most on the object side of the overall lens system to the imaging surface of the solid-state imaging element) is short.
A zoom lens that is conceivably suited to meet these demands is a two-group zoom lens constituted by a first lens group having a negative power and a second lens group having a positive power arranged in that order from the object side to the image plane side, which performs zooming by changing the spacing between these two lens groups. However, a characteristic of such zoom lenses composed of two lens groups is that they are suited for wide angles, and thus have the problem of a small zoom ratio of about ×2. Also, to perform focus adjustment, it is necessary to move at least one of the two lens groups, but since both lens groups are large and heavy, there is also the problem that two-group zoom lenses as described above are not suited for autofocus. Accordingly, to solve these problems, many proposals have been made for zoom lenses constituted by three lens groups, in which a third lens group having a positive power is disposed on the image plane side of a zoom lens having two lens groups.
For example, JP H11-194274A discloses a zoom lens constituted by three lens groups, these being a first lens group, arranged in that order from the object side in a negative, positive power arrangement, a second lens group constituted by four lenses, and a third lens group constituted by a single lens. Also, JP 2001-296475A discloses a zoom lens constituted by three lens groups, these being a first lens group, arranged in that order from the object side in a negative, positive power arrangement or in a negative, negative, positive power arrangement, a second lens group constituted by four lenses, and a third lens group constituted by a single lens or a single cemented lens.
These zoom lenses constituted by three lens groups are made of a first lens group having a negative power, a second lens group having a positive power, and a third lens group having a positive power, arranged in that order from the object side to the image plane side. When zooming from the wide-angle end to the tele end, the air space between the first lens group and the second lens group is monotonically decreased and the air space between the second lens group and the third lens group is monotonically increased and the third lens group is moved. Moving the third lens group in the direction of the optical axis carries out focus adjustment. The third lens group improves the telecentricity. Here, the third lens group is made of a single lens or a single cemented lens with a small outer diameter and can be driven at high speeds using a low-power compact motor, and thus it is suited as a lens group for focus adjustment in autofocus, in which high-speed movement is required. The movement of the first lens group and the second lens group is carried out using cylindrical cams. Consequently, a collapsing configuration in which all three lens groups are drawn toward the solid-state imaging element using cylindrical cams when the zoom lens is not in use can be adopted. Also, if such a zoom lens is used in an electronic still camera, then the electronic still camera can be made thin in the depth direction when not in use.
Among video cameras, those provided with a camera shake-correction function for correcting vibration of the captured image when the user's camera shakes have been released on the market. Many methods have been proposed for camera shake correction, and methods for parallel displacing some of the lens groups of the zoom lens in the direction perpendicular to the optical axis are gradually being adopted (for example, JP 2000-298235A).
JP H11-52245A discloses a zoom lens made of a first lens group having a negative power, a second lens group having a positive power, a third lens group having a positive power, and a fourth lens group having a positive power or a negative power, arranged in that order from the object side to the image plane side, wherein correction of camera shake is carried out by parallel displacement of the third lens group in the direction perpendicular to the optical axis. This publication also discloses that decentering curvature of field and decentering coma when the third lens group is parallel displaced for correction of camera shake can be favorably corrected.
With compact type electronic still cameras, from the standpoint of ease of carrying, it is preferable that they are thin in the depth direction when not in use. It is also preferable that the captured images are made high resolution.
To make an electronic still camera thin in the depth direction when not in use, it is possible to reduce the picture plane size of the solid-state imaging element and to shorten the optical total length of the zoom lens when not in use. Also, to shorten the optical total length when the zoom lens is not in use, the zoom lens can be given a collapsing configuration and the overall length of each of the lens groups can be shortened so as to shorten the distance between the lens groups when collapsed.
To make the images captured by an electronic still camera high resolution, it is necessary to increase the pixel number of the solid-state imaging element and make its zoom lens high resolution.
However, reducing the picture plane size of the solid-state imaging element and increasing the pixel number significantly reduces the pixel pitch, and thus it is necessary to be careful that the image-forming properties of the zoom lens are deteriorated due to the effects of diffraction. To reduce the effect of diffraction, the F-number of the zoom lens can be made small.
Also, taking into account the fact that the peripheral portion of the captured image may be cut away, it is preferable that the entire picture resolution of the captured image is more uniform. Solid-state imaging elements have good resolution uniformity, but the resolution of the zoom lens generally tends to be high at the center of the image plane and low at the peripheral portions of the image plane.
Also, in the zoom lens discussed in JP H11-194274A, the distortion is small but both the curvature of field in the sagittal direction and the curvature of field in the meridional direction are large. Thus, the zoom lens described in this publication has the problem that the image-forming properties at the peripheral portions of the image plane are not good. Also, the zoom lenses disclosed in JP 2001-296475A and JP H2001-296476A have the problem that it is difficult to achieve good resolution at the peripheral portions of the image plane due to sagittal flare that occurs at the peripheral portions of the image plane.
Zoom lenses for electronic still cameras have the problem that, compared to zoom lens used in 35 mm film cameras, the production tolerance of the lens elements and the assembly tolerance of the zoom lens unit are very severe. This is due to the fact that the diagonal length of the effective picture area of a solid-state imaging element is significantly smaller than the 43.3 mm diagonal length of the effective picture area (36 mm horizontal×24 mm vertical) of the 35 mm film camera. Also, to achieve a collapsed configuration, a moving lens barrel that moves during zooming and a stationary lens barrel that supports the moving lens barrel are required, but when the optical total length when in use is much longer than the optical total length when collapsed, the stationary lens barrel cannot stably support the moving lens barrel, and thus a portion of the lens groups becomes decentered, leading to the problem of a deteriorated ability to form the captured image. For that reason, the design performance of the zoom lens is good, but since the production tolerance and the assembly tolerance of the lens elements and the lens barrel components are extremely severe, there is the problem that it is difficult to achieve image-forming property that is near the design performance through mass production.
The zoom lens disclosed in JP H11-52245A has a function for correcting camera shake, but it has the problem that the overall length of the second lens group is long due to the incorporation of a long air distance or a thick lens, and thus even if a collapsed configuration is adopted, the optical total length when collapsed is not particularly short. Also, the zoom lens discussed in this publication is made of ten or eleven lenses, and due to the large number of lenses, there is also the problem of increased costs.